1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a radio for use in wireless communications and an apparatus and method which uses a radio, modem and controller for implementing wireless communications.
2. Description of the Related Art
Conventional radios used in wireless communications, such as radios used in conventional cellular telephones, typically include several discrete RF circuit components. This results in such radios having a large size and footprint, being expensive and power consuming. In order to illustrate this, it is useful to analyze a conventional implementation of circuitry that could be used, for example, as the receiver portion of a conventional radio. Specifically, a traditional receiver architecture may employ superhetrodyne techniques as shown in FIG. 1. In a superhetrodyne architecture an incoming signal is frequency translated from its radio frequency (RF) to a lower intermediate frequency (IF). The signal at IF is subsequently translated to baseband where further digital signal processing or demodulation may take place. Receiver designs may have multiple IF stages. The reason for using such a frequency translation scheme is that circuit design at the lower IF frequency is much more manageable for signal processing. It is at these IF frequencies that the selectivity of the receiver is implemented, automatic gain control (AGC ) is introduced, etc.
In addition to more manageable circuit design, high Q (i.e., high xe2x80x9cquality factorxe2x80x9d) filters are also easier to implement at IF. High Q filters are used to meet the selectivity and spurious rejection requirements dictated by wireless systems. Surface acoustic wave (SAW) and ceramic technology are typically used for the filtering depending on the frequency of operation. Although these respective technologies have improved in terms of size and performance they are still relatively large. Moreover, due to the relatively high frequency of the most IFs, it is not realistic, yet, to implement this filter using integrated circuit (IC) techniques.
As an alternative to the superhetrodyne techniques, a direct conversion receiver architecture may be used. This is shown in FIG. 2. This scheme translates the incoming RF signal directly to baseband. The direct conversion architecture has several advantages. First, there is no need for the high-Q filters required for traditional superhetrodyne architecture. Generally, all that is needed is a broadband RF filter which is used to reduce the dynamic range requirements of the RF down-converter. Second, there are a limited number of RF circuit blocks. Third, oscillators may be reduced to one. Fourth, it offers the smallest size solution since bulky off-chip filters are no longer required. Finally, because the low-pass channel filters are readily integrated, a fully integrated solution is achievable.
Although the direct conversion receiver architecture has several advantages, there are several practical implementation problems. In general, wireless communications devices use high-frequency signals: 900 MHz to 1900 MHz for cellular phones and higher (up to 6 GHz) for other systems, such as wireless LANs. Radios for the so called xe2x80x9cBluetooth standardxe2x80x9d (discussed below) operate in the unlicensed ISM band at 2.4 GHz. Signals at such frequencies are difficult to generate and control. They also have a tendency to interfere with each other, as they are easily coupled by parasitic properties present in all electronic components, including integrated circuits. In ICs, many of the undesirable parasitic effects result from the conductive silicon substrate on which the circuits are fabricated.
Specifically, in the direct conversion receiver of FIG. 2, due to limited local oscillator (LO) to RF isolation in the down-converter, and limited reverse isolation in the low noise amplifier (LNA) 10, an amount of LO signal can appear at the output of the receiver and effectively be transmitted at the antenna. Wireless regulatory authorities limit the amount of spurious signal that can be radiated by the receiver, so limiting the amount of LO radiation is necessary to meet these specifications. In addition, LO leakage causes particular problems for direct conversion receivers. The lack of LO isolation causes self mixing in the direct down converter that manifests as a DC offset at baseband.
Specifically, there are several mechanisms through which LO leakage may occur. For example, there may be conducted paths between components. This occurs because there is limited isolation from the LO port of the mixers 12 to the RF port of the mixers 12. There is also limited reverse isolation through the low-noise amplifying stages preceding the mixers 12. A parasitic signal path for signals through the substrate, as well as a lateral signal path through the substrate, can also occur. In addition to the conducted path, there may also be radiated paths via the bond wires used to interconnect the circuit blocks to the outside world. The bond wires act as antennas and couple RF energy, such as that of the LO, to adjacent pins.
The traditional solution for reducing the amount of signal that appears at the antenna port is to have the LO, i.e., the voltage controlled oscillator (VCO) 14, at a different frequency than the incoming RF signal, as is indicated in FIG. 2. This utilizes the filtering effects of matching, etc., to reduce the amount of LO leakage. This solution, however, requires the use of dividers or multipliers 16, as shown in FIG. 2, which adds additional circuitry. Furthermore, this solution does not solve all of the problems of LO leakage associated with direct conversion receivers.
The market requirements for today""s mobile communication terminals are such that wireless product manufacturers have gone to smaller and smaller form factors with improved performance and lower cost. This has resulted in radio designers, for both circuits and systems, looking for ways of accommodating these requirements. Therefore, it follows that it would be highly desirable to have an improved radio design that is a low cost, low power and small size solution, and that overcomes the disadvantages discussed above. Such an improved radio design would have many uses in wireless communications, including for example, use in cellular telephones, cordless telephones, personal computer (PC) interconnections, etc.
With respect to PC interconnections, at present, standard wire interconnects are used to link together PC based products, such as laptop and notebook computers and personal digital assistants (PDAs). For example, RS232and Universal Serial Bus (USB) are commonly used standards that are offered as connections on many devices. Some wireless interconnects are also being used, such as infrared (IR). IR suffers from the disadvantage of being somewhat directional in its ability to communicate with other IR devices. It has been predicted that in the near future there will be a convergence of traditional wireless and computer technologies, such as cellular phones, and PC based products. One key to a successful implementation of a standard that facilitates this convergence is to make it almost effortless for the user to use.
There are several wireless communications standards either in existence or being proposed, such as for example, Home RF, IEEE 802.11, etc. One wireless communications standard that is currently being proposed is the xe2x80x9cBluetoothxe2x80x9d standard. Bluetooth is a global specification for wireless connectivity. It is based on a low-cost, short-range radio link that enables wireless communication of data and voice and facilitates protected ad hoc wireless connections for stationary and mobile communication environments. The proposal of Bluetooth is to offer a solution that yields rugged wireless connectivity. The Bluetooth standard will be discussed herein as an example of a wireless communications standard, but it should be understood that the teachings of the present invention may be applied to any type of wireless communications and is not limited to the Bluetooth standard.
The Bluetooth Specification
The Bluetooth standard is being developed through the contributions of the members of the Bluetooth Special Interest Group (SIG). The Bluetooth specification, the contents of which are hereby incorporated by reference, is available from the Bluetooth Special Interest Group. Information regarding the Bluetooth standard, as well as procedures for obtaining the latest version of the Bluetooth specification, is available at the Internet web site http://www.bluetooth.com. As set forth therein, Bluetooth technology allows for the replacement of the many proprietary cables that connect one device to another with one universal short-range radio link. For instance, Bluetooth radio technology built into both the cellular telephone and the laptop would replace the cumbersome cable used today to connect a laptop to a cellular telephone. Printers, PDA""s, desktops, fax machines, keyboards, joysticks and virtually any other digital device can be part of the Bluetooth system. But beyond untethering devices by replacing the cables, Bluetooth radio technology provides a universal bridge to existing data networks, a peripheral interface, and a mechanism to form small private ad hoc groupings of connected devices away from fixed network infrastructures. Designed to operate in a noisy radio frequency environment, the Bluetooth radio uses a fast acknowledgment and frequency hopping scheme to make the link robust. Bluetooth radio modules avoid interference from other signals by hopping to a new frequency after transmitting or receiving a packet. Compared with other systems operating in the same frequency band, the Bluetooth radio typically hops faster and uses shorter packets. This makes the Bluetooth radio more robust than other systems. Short packages and fast hopping also limit the impact of domestic and professional microwave ovens. Use of Forward Error Correction (FEC) limits the impact of random noise on long-distance links. The encoding is optimized for an uncoordinated environment.
Bluetooth radios operate in the unlicensed ISM band at 2.4 GHz. A frequency hop transceiver is applied to combat interference and fading. A shaped, binary FM modulation is applied to minimize transceiver complexity. The gross data rate is 1 Mb/s. A Time-Division Duplex scheme is used for full-duplex transmission.
The Bluetooth baseband protocol is a combination of circuit and packet switching. Slots can be reserved for synchronous packets. Each packet is transmitted in a different hop frequency. A packet nominally covers a single slot, but can be extended to cover up to five slots. Bluetooth can support an asynchronous data channel, up to three simultaneous synchronous voice channels, or a channel which simultaneously supports asynchronous data and synchronous voice. Each voice channel supports 64 kb/s synchronous (voice) link. The asynchronous channel can support an asymmetric link of maximally 721 kb/s in either direction while permitting 57.6 kb/s in the return direction, or a 432.6 kb/s symmetric link.
A piconet is a collection of devices connected via Bluetooth technology in an ad hoc fashion. A piconet starts with two connected devices, such as a portable PC and cellular phone, and may grow to eight connected devices. All Bluetooth devices are peer units and have identical implementations. Each unit has its own unique 48-bit address referred to as the Bluetooth device address. However, when establishing a piconet, one unit will act as a master and the other(s) as slave(s) for the duration of the piconet connection. Multiple independent and non-synchronized piconets form a scatternet. A master unit is the device in a piconet whose clock and hopping sequence are used to synchronize all other devices in the piconet. All devices in a piconet that are not the master are slave units. An Active Member address is a 3-bit address to distinguish between units participating in the piconet. The master unit receives the all zero Active Member for itself and thus there can be only seven active slaves in a piconet at any given time. Parked units are devices in a piconet which are synchronized but do not have Active Member addresses but can have 8-bit Passive Member addresses or be addressed with the full Bluetooth address. Active Member devices in a piconet can enter power-saving modes in which device activity is lowered. This called a sniff and hold mode.
The Bluetooth system supports both point-to-point and point-to-multi-point connections. Several piconets can be established and linked together ad hoc, where each piconet is identified by a different frequency hopping sequence. All users participating on the same piconet are synchronized to this hopping sequence and the master""s Bluetooth device address. The topology can best be described as a multiple piconet structure. The full-duplex data rate within a multiple piconet structure with 10 fully-loaded, independent piconets is more than 6 Mb/s. This is due to a data throughput reduction rate of less than 10% according to system simulations based on 0 dBm transmitting power (at the antenna).
Voice channels use the Continuous Variable Slope Delta Modulation (CVSD) voice coding scheme, and never retransmit voice packets. The CVSD method was chosen for its robustness in handling dropped and damaged voice samples. Rising interference levels are experienced as increased background noise: even at bit error rates up 4%, the CVSD coded voice is quite audible.
Referring to FIG. 3, there is illustrated the different functional blocks in the Bluetooth system. The different functions in the Bluetooth system are: a radio 20, a link baseband controller (LC) 24, a link manager (LM) 26, software functions 28, and a host processor or controller 30. The radio 20 is hardware that translates between binary bits and radio signals received and transmitted from an antenna 22. The LC 24 is hardware and/or software for performing the baseband processing and basic protocols close to the physical layer. The LM 26 is software that carries out protocols such as link setup, authentication, link configuration, control, etc. The software functions 28 may include configuration and diagnosis utility, device discovery, cable emulation, peripheral communication, audio communication and call control, object exchange for business cards and phone books, and networking protocol.
With respect to the radio 20, the Bluetooth air interface is based on a nominal antenna power of 0 dBm. The air interface complies with the FCC rules for the ISM band at power levels up to 0 dBm. Spectrum spreading has been added to facilitate optional operation at power levels up to 100 mW worldwide. Spectrum spreading is accomplished by frequency hopping in 79 hops displaced by 1 MHz, starting at 2.402 GHz and stopping at 2.480 GHz. Due to local regulations the bandwidth is reduced in Japan (2.471-2.497 GHz), France and Spain. This is handled by an internal software switch. For most functions, the maximum frequency hopping rate is 1600 hops/s. For paging functions, the hopping rate is 3200 hops/s. The nominal link range is 10 centimeters to 10 meters, but can be extended to more than 100 meters by increasing the transmit power.
With respect to establishing network connections, before any connections in a piconet are created, all devices are in STANDBY mode. In this mode, an unconnected unit periodically xe2x80x9clistensxe2x80x9d for messages every 1.28 seconds. Each time a device wakes up, it listens on a set of hop frequencies defined for that unit. The number of hop frequencies varies in different geographic regions; 32 is the number for most countries (except Japan, Spain and France where it is currently limited to 16).
The connection procedure is initiated by any of the devices which then becomes master. A connection is made by a PAGE message if the address is already known, or by an INQUIRY message followed by a subsequent PAGE message if the address is unknown.
In the initial PAGE state, the master unit will send a train of 16 identical page messages on 16 different hop frequencies defined for the device to be paged (slave unit). If there is no response, the master transmits a train on the remaining 16 hop frequencies in the wake-up sequence. If the radio link is reliable, the maximum delay before the master reaches the slave is twice the wakeup period (2.56 seconds) while the average delay is half the wakeup period (0.64 seconds).
The INQUIRY message is typically used for finding Bluetooth devices, including public printers, fax machines and similar devices with an unknown address. The INQUIRY message is very similar to the page message, but may require one additional train period to collect all the responses.
A power saving mode can be used for connected units in a piconet if no data needs to be transmitted. The master unit can put slave units into HOLD mode, where only an internal timer is running. Slave units can also demand to be put into HOLD mode. Data transfer restarts instantly when units transition out of HOLD mode. The HOLD is used when connecting several piconets or managing a low power device such as a temperature sensor.
Two more low power modes are available, the SNIFF mode and the PARK mode. In the SNIFF mode, a slave device listens to the piconet at reduced rate, thus reducing its duty cycle. The SNIFF interval is programmable and depends on the application. In the PARK mode, a device is still synchronized to the piconet but does not participate in the traffic. Parked devices have given up their Active Member address and occasionally listen to the traffic of the master to re-synchronize and check on broadcast messages.
If the modes are listed in increasing order of power efficiency, the SNIFF mode has the higher duty cycle, followed by the HOLD mode with a lower duty cycle, and finishing with the PARK mode with the lowest duty cycle.
The link type defines what type of packets can be used on a particular link. The Bluetooth baseband technology supports two link types: Synchronous Connection Oriented (SCO) type (used primarily for voice), and Asynchronous Connectionless (ACL) type (used primarily for packet data).
Different master-slave pairs of the same piconet can use different link types, and the link type may change arbitrarily during a session. There are sixteen different packet types. Four of these are control packets and are common for both SCO and ACL links. Both link types use a Time Division Duplex (TDD) scheme for full-duplex transmissions. The SCO link is symmetric and typically supports time-bounded voice traffic. SCO packets are transmitted at reserved slots. Once the connection is established, both master and slave units may send SCO packets without being polled. One SCO packet type allows both voice and data transmissionxe2x80x94with only the data portion being retransmitted when corrupted. The ACL link is packet oriented and supports both symmetric and asymmetric traffic. The master unit controls the link bandwidth and decides how much piconet bandwidth is given to each slave, and the symmetry of the traffic. Slaves must be polled before they can transmit data. The ACL link also supports broadcast messages from the master to all slaves in the piconet.
There are three error-correction schemes defined for Bluetooth baseband controllers: 1/3 rate forward error correction code (FEC); 2/3 rate forward error correction code FEC; and Automatic repeat request (ARQ) scheme for data.
The purpose of the FEC scheme on the data payload is to reduce the number of retransmissions. However, in a reasonably error-free environment, FEC creates unnecessary overhead that reduces the throughput. Therefore, the packet definitions have been kept flexible as to whether or not to use FEC in the payload. The packet header is always protected by a 1/3 rate FEC; it contains valuable link information and should survive bit errors. An unnumbered ARQ scheme is applied in which data transmitted in one slot is directly acknowledged by the recipient in the next slot. For a data transmission to be acknowledged both the header error check and the cyclic redundancy check must be okay; otherwise a negative acknowledge is returned.
With respect to the authentication and privacy, the Bluetooth baseband provides user protection and information privacy mechanisms at the physical layer. Authentication and encryption are implemented in the same way in each Bluetooth device, appropriate for the ad hoc nature of the network. Connections may require a one-way, two-way, or no authentication. Authentication is based on a challenge-response algorithm. Authentication is a key component of any Bluetooth system, allowing the user to develop a domain of trust between a personal Bluetooth device, such as allowing only the owner""s notebook computer to communicate through the owner""s cellular telephone. Encryption is used to protect the privacy of the connection. Bluetooth uses a stream cipher well suited for a silicon implementation with secret key lengths of 8 to 128 bits. Key management is left to higher layer software.
The goal of Bluetooth""s security mechanisms is to provide an appropriate level of protection for Bluetooth""s short-range nature and use in a global environment. Users requiring stalwart protection are encouraged to use stronger security mechanisms available in network transport protocols and application programs.
Conventional Implementations of Bluetooth
In a conventional implementation of the Bluetooth system, the modulator and demodulator are typically implemented in a combination of the radio 20 and LC 24. The hardware for the LC 24 will typically be specific to the radio 20 being used. The LC 24 is specific to the system since it implements system protocol functions. It is also specific to the type of modulation used in the system and it is also dependent upon the type of radio architecture implemented. At least some of this hardware will typically be integrated into a baseband IC performing LC 24 and possibly LM 26 functions as an application specific integrated circuit (ASIC). A demodulator will demodulate the signal that appears at the output of the radio receiver. The output from the demodulator is raw data that is typically passed directly to the LC 24 for further processing to correct any errors and extract the payload. The output from the host processor 30 is passed through Bluetooth Software Functions 28 and LM 26 to the LC 24. The signal is data that has been formatted by the LC 24 into the appropriate structure with the addition of error correction, address bits, etc.
This conventional implementation has several disadvantages. Specifically, it requires a special ASIC with a processor to implement all of the Bluetooth functions, and the processor runs to perform all operations. All received data goes from the radio to the LC 24 over an external interface which will consume power and cause interference. The LC 24 has to perform frequency hopping by sending many commands to the radio. Finally, the LC 24 has to have many I/O lines into the radio for monitor and control. It would be desirable to have a more flexible way to implement the Bluetooth system, as well as a lower cost, lower power solution which conserved the resources of the host controller or microprocessor 30.
With respect to the Bluetooth radio 20, a conventional implementation will typically include several discrete RF circuit components, which as discussed above, will result in the radio 20 having a large size and footprint, being expensive and power consuming.
Therefore, an improved radio design that is a low cost, low power and small size solution could be used not only in cellular telephones, cordless telephones, etc., but also in implementing various wireless communications standards, specifications and/or protocols such as the Bluetooth standard. Furthermore, it would also be highly desirable to have an apparatus and method for wireless communications that could be used to efficiently implement various wireless communications standards, specifications and/or protocols, such as the Bluetooth standard, into a flexible, low cost, low power and small size solution.
The present invention provides an apparatus for receiving signals. The apparatus includes a low noise amplifier (LNA) configured to receive a radio frequency (RF) signal. An I/Q direct down converter is coupled to the LNA. The I/Q direct down converter is configured to split the RF signal into real and imaginary components and to down convert the real and imaginary components directly to baseband signals. A local oscillator (LO) is coupled to the I/Q direct down converter and is configured to drive the I/Q direct down converter. First and second filters are coupled to the I/Q direct down converter. The first and second filters are configured to filter the down converted real and imaginary components, respectively. First and second analog-to-digital converters (ADCs) are coupled to the first and second filters, respectively. The first and second ADCs are configured to convert the real and imaginary components into digital signals. The first and second ADCs have a dynamic range that is wide enough to convert the filtered, down converted real and imaginary components to digital signals without using variable gain on the filtered and down converted real and imaginary components.
The present invention also provides an apparatus for receiving and transmitting signals. The apparatus includes a local oscillator (LO), a direct conversion transmitter coupled to the LO that is configured to up-convert baseband signals directly to radio frequency (RF) for transmission, and a direct conversion receiver coupled to the LO that is configured to down-convert a received RF signal directly to baseband. The direct conversion receiver includes an analog-to-digital converter (ADC) that is configured to convert the down-converted received RF signal into a digital signal. The ADC has a dynamic range that is wide enough to convert the down-converted received RF signal to a digital signal without using variable gain on the down-converted received RF signal.
The present invention provides an apparatus for use in wireless communications. The apparatus includes a radio, a modem and a controller integrated onto a single integrated circuit (IC). The radio has a receiver for receiving data and a transmitter for transmitting data. The modem is coupled to the radio and is configured to demodulate received data and modulate data for transmission. The controller is coupled to the modem and includes a digital interface for external communications through which received data and data for transmission is sent, a connection state machine configured to accept commands through the digital interface and to respond to the commands by initiating a sequence, and a receive/transmit state machine configured to perform state control of the radio in response to the initiated sequence.
The present invention also provides a method of performing wireless communications. The method includes receiving commands from an external processor through a digital interface where the digital interface is integrated onto a single integrated circuit (IC); initiating a sequence in circuitry included on the single IC in response to the received commands; performing state control of a radio that is integrated onto the single IC in response to the initiating sequence; and communicating data with the radio.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized.